1. Field of the invention
This invention relates to an apparatus, such as an optical printer, for optically writing information. The invention also relates to an apparatus, such as an optical image reader, for optically detecting or reading information.
2. Description of the Prior Art
Generally, optical printers include a row of light emitting diodes (LED) arranged to face a photosensitive drum in parallel thereto, and a lens system arranged between the diode row and the drum. The row of diodes, when driven for lighting, forms a row of luminescent dots in various patterns depending on the driving control. The lens system serves to form an image of the luminescent dot row on the drum in rotation, thus effecting intended printing in accordance with the driving control.
Most commonly, the lens system for the optical printer is constituted by an array of self-focusing microlenses commercially available for example from NIPPON SHEET GLASS CO., LTD. The array of self-focusing microlenses is designed and arranged to form an actual size image of a luminescent dot row on the photosensitive drum without image inversion. In other words, the dot image row on the photosensitive drum exactly corresponds in size and dot arrangement to the actual luminescent dot row. Therefore, in order to conduct correct printing with high quality, it is necessary to arrange the LEDs at constant minute spacing throughout the entire length of the LED row.
As shown in FIG. 23 of the accompanying drawings, on the other hand, the LED row, which can be very long in some applications, is constituted usually by arranging a plurality of LED array chips 100 in intimate end-to-end contact with each other. Each array chip 100 incorporates an array of LEDs 100a at constant minute spacing. Thus, the spacing y between each two adjacent LEDs 100a within each single array chip 100 must be exactly equal to the spacing x between two adjacent end LEDs of two different array chips in order to make the dot spacing constant over the entire length of the LED row, as required to ensure high printing quality. However, it is in fact virtually impossible to satisfy this spacing requirement for the following reasons.
First, in manufacture of LED array chips, each unit chip is diced or cut from a long wafer carrying a number of LEDs. However, inevitable limitations on dicing accuracy result in that a dot spacing irregularity occurs at the position where each two adjacent array chip 100 are held in intimate end-to-end contact. An attempt to increase the dicing accuracy to an acceptable level, on the other hand, will lead to an unacceptable cost increase.
Second, a similar spacing irregularity also occurs due to mounting errors in bonding the array chips onto a substrate. Further, mounting the array chips in intimate end-to-end contact inheretly involves the risk of arrangement disorder, chip damaging and chip contamination.
In this way, the prior art optical printer incorporating the row of constantly spaced LEDs and the array of self-focusing microlenses is disadvantageous in the difficulty of ensuring high printing quality. Further, the prior art printer is also disadvantageous in that the self-focusing microlens array is relatively expensive.
Obviously, the problems discussed above hold with respect to conventional optical image sensors or readers wherein a plurality of sensor array chips are arranged in a row so that all image sensing elements (light receiving elements) are constantly spaced over the entire length of the chip row.